Method and apparatus for relative positioning of a frequency, in particular an optical frequency

ABSTRACT

In a method of the invention an emission wave (F(2P-1)) which is to be positioned within the spectrum relative to an external wave (F(2P-2)B) is modulated. Said external wave is mixed with one of the side waves that results from said modulation (F(2P-1)A) and the emission wave is positioned in response to variations in the frequency of a beat signal (F(2P-2)BE) that results from said mixing. The invention applies in particular to implementing an optical fiber communications network.

The present invention is applicable when a controlled frequency referredto as the "emission" frequency is to be brought or maintained to or at aposition within the frequency spectrum, which position is situated at adefined spectrum distance from another frequency referred to as the"support" frequency which is defined elsewhere and which may bevariable.

The advantages of the invention appear, in particular, when the emissionfrequency and the support frequency both lie in a range of the opticalspectrum and when said defined spectrum distance is such as to make itdifficult or expensive to produce, amplify, or process a beat signal ata frequency equal to said distance by means of components that arecommercially available.

The invention is more particularly applicable to certain optical fibercommunications networks that transmit a plurality of messagessimultaneously. In such networks, messages are transmitted in the formof modulation applied to waves whose respective frequencies correspondto respective ones of said emission frequencies each of which is to bepositioned relative to at least one other one of said frequencies, whichfrequency then constitutes said support frequency for positioning saidemission frequency.

BACKGROUND OF THE INVENTION

In more general terms, known frequency positioning apparatus includesvarious items that are common, as to their functions specified below,with apparatus of the present invention, namely:

an emitter for emitting an emission wave at a controlled emissionfrequency; and

emission position servo-control means including a positioning mixer thatperforms wave mixing to form a positioning beat signal and that controlssaid emitter in response to variations in the frequency at a definedspectrum distance position said emission frequency at a defined spectrumdistance from a defined support frequency.

It will be understood that the word "wave" as used above is equallyapplicable to a wave that is propagating freely, to a wave that isguided, or to a signal that is propagating along a transmission line,and that the nature of said wave or signal may be electromagnetic,acoustic, etc. . . . .

Such apparatus is included in each terminal of a known communicationsnetwork described in European patent document EP-A-0 381 102 (F°16761).In such a terminal, a fraction of a locally generated optical emissionwave is transmitted to one input of a positioning mixer whose otherinput receives other waves coming from other terminals, with thefrequency of one of said other waves constituting said supportfrequency.

An object of the present invention is to improve relative positioning ofa frequency.

Another object is to facilitate implementing a frequency-multiplexedcommunications network in which frequencies that are adjacent in asuccession of emission frequencies are to be separated by a frequencyincrement, and in which said frequency increment must be large in orderto enable messages to be transmitted at high data rates.

SUMMARY OF THE INVENTION

In a method of the invention, a side signal is generated by modulatingan emission signal having an emission frequency (F(2P-1)) which is to bepositioned in the spectrum relative to an external signal (F(2P-2)B),the external signal is mixed with the side signal (F(2P-1)A), and theemission frequency is positioned in response to variations in thefrequency of a beat signal (F(2P-2)BE) that results from said mixing.

Apparatus of the invention includes the above-mentioned common items andsaid emission position servo-control means include modulation means toapply modulation at a positioning assistance frequency (FS) to at leasta fraction of said emission wave (F(2P-1)), thereby providing at leastone first positioning assistance side wave (F(2P-1)A) whose frequency isat a spectrum distance from said emission frequency equal to saidpositioning assistance frequency;

said positioning mixer mixing said first positioning assistance sidewave with an external emission positioning wave (F(2P-2)B)representative of said support frequency (F(2P-2)) to form saidpositioning beat signal (F(2P-2)BE).

Such a method and such apparatus present an advantage in whenever a beatfrequency equal to the difference between the emission frequency and thesupport frequency does not have a value that is convenient forestablishing a control frequency in response to which the emissionfrequency can be controlled. Under such circumstances, the presentinvention is performed by making an appropriate choice for thepositioning assistance frequency and an appropriate selection of one ofthe beat signals, e.g. by means of a frequency filter, selected from thetwo beat signals that are formed by the positioning assistancemodulation. These choices and this selection are performed so that thefrequency of the beat signal formed and selected in this way enables theemission frequency to be positioned better.

The improvement relates to at least one useful quality of thepositioning. Examples of such qualities are the following: speed,accuracy, sensitivity, reliability, low implementation cost, small size,low energy consumption, etc. . . . .

Apparatus of the present invention may also include the followingadvantageous dispositions:

said modulation of the emission wave (F(2P-1)) by a positioningassistance frequency (FS) forms two positioning assistance side waves(F(2P-1)A), (F(2P-1)B) disposed in the spectrum on either side of saidemission wave, said first positioning assistance side wave (F(2P-1)A)being that one of these two waves which is closer to said supportfrequency (F(2P-2));

said positioning assistance frequency is less than a frequency incrementwhich constitutes said defined spectrum distance;

said apparatus includes second modulation means for applying modulationat a second positioning assistance frequency to at least a fraction of asupport wave having said support frequency (F(2P-2)), thereby formingone of said external emission positioning waves whose frequency(F(2P-1)B) is closer to said emission frequency (F(2P-1)) then saidsupport frequency and is situated at a spectrum distance from saidsupport frequency equal to said second positioning assistance frequency;and

said second positioning assistance frequency is equal to saidpositioning frequency, said frequencies being less than half saidfrequency increment, and preferably greater than one-fourth of saidincrement.

The present invention may be advantageously implemented, in particular,in a frequency multiplexed optical fiber communications network.

In such a network, the emission frequencies that are simultaneously inuse form a sequence called a "stack" and extending from a frequency atthe bottom of the stack to a frequency at the top. Terminals in thenetwork emit emission waves at said frequencies and form a correspondingsequence. These frequencies are separated from one another by not lessthan said frequency increment. In the network, said first-mentionedmodulation means belong to a terminal "under consideration" whoseemission frequency constitutes the above-mentioned emission frequency.The said second modulation means belong to a terminal preceding theterminal under consideration in said sequence and whose emissionfrequency constitutes the above-mentioned support frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is an overall view common, in particular, both to a first datanetwork and to a second data network given as examples ofimplementations of the present invention.

FIG. 2 is a block diagram of a terminal in the first network.

FIG. 3 is a block diagram of a control unit belonging to said network.

FIG. 4 shows the spectrum of various optical waves present on the firstnetwork.

FIG. 5 shows the spectrum of various optical waves present on thereception fiber of a terminal belonging to the first network.

FIG. 6 shows the spectrum of the optical waves of FIG. 5 after saidwaves have been mixed with the optical waves emitted by the emitter inthe same terminal.

FIG. 7 shows the spectrum of electrical signals that results fromdetecting the optical waves of FIG. 6 to enable the spectrum position ofthe emitter in the terminal of FIG. 2 to be servo-controlled.

FIG. 8 shows the spectrum of the optical waves of FIG. 5 after thesewaves have been mixed with the optical wave emitted by the localoscillator in the terminal of FIG. 2.

FIG. 9 shows the spectrum of the electrical signals that result fromdetecting the optical signals of FIG. 8 to enable the frequency of thelocal oscillator of the terminal of FIG. 2 to be servo-controlled.

FIG. 10 is a block diagram of a terminal in a second network given as anexample of an implementation of the present invention.

DETAILED DESCRIPTION

With reference to FIG. 1, the present description begins by recallingthe general structure common to networks of this kind. Such a networkcomprises the following items:

a plurality of terminals (T1, T2, T3, . . . , TN) associated withrespective user peripherals between which messages are to betransmitted;

two optical fibers associated with each of said terminals (T1) forguiding optical waves, said fibers comprising an emission fiber (61) forguiding emission waves emitted at an emission frequency of the terminaland a reception fiber (62) for guiding waves that are to be received bythe terminal; and

a star coupler (CE) for receiving the optical waves that reach it viasaid emission fibers, and for transmitting each of said optical waves toall of said terminals via said reception fibers.

A control unit BG includes a base generator (B1) which is shown in FIG.3 and which emits an optical base wave at a base frequency (FO) toconstitute a frequency reference for controlling said emissionfrequencies.

In such a network, each of the terminals is sometimes free and sometimesbusy. When it is busy, it may be precalling and/or precalled and then itmay be calling or called.

A terminal is calling and emitting while it is transmitting a message toa called terminal in the form of information-carrying modulation on anoptical carrier wave which constitutes an emission wave from saidterminal and which is emitted for a call duration. It is assumed belowthat the frequency specific to said carrier wave constitutessimultaneously an emission frequency and spectrum position of saidterminal, and a transmission frequency for said message. It can bevaried on command over a spectrum range of the network. Its positionwithin said range may be defined on the basis of the base frequency.Below, when it is said that said transmission frequency is, at a giveninstant, more or less close or more or less distant or that in thecourse in time it moves closer or it moves away, the description relatesto the closeness or the distance of said transmission frequency relativeto said base frequency. To simplify some of the explanation, the casewhere the base frequency is lower than the emission frequencies issometimes considered more particularly, and words such as "higher" and"lower" applied to frequencies relate to such circumstances. Thefrequencies of the modulated emission wave lie in a message channelwhich is reserved within the frequency range of the network fortransmitting a message, and which for said purpose has a messagespectrum width. This wave is received by the called terminal andconstitutes for said terminal a reception wave whose frequencyconstitutes a reception frequency. The terminal is then receiving. Thecalling and the called terminals are in pairs forming respective pairsof terminals in communication with each other.

Communication may be one-way. A single message is then transmitted. Itis transmitted from the calling terminal to the called terminal.Communication may also be two-way. Under such circumstances, it may beperformed in so-called "alternating" mode. The calling terminal and thecalled terminal then take up the same position in the spectrum and thesame message channel is occupied by go messages and by return messageswhich follow one another in time. Two-way communication may also beperformed in so-called "duplex" mode. Under such circumstances, theabove-mentioned message then constitutes a go message occupying a gomessage channel. The called terminal is suitable for responding to saidmessage by emitting a return message during the same call period. Thereturn message is transmitted to the calling terminal on an opticalcarrier wave which occupies a return message channel and whichconstitutes an emission wave for the called terminal and a receptionwave for the calling terminal. A call is then constituted by the go andreceive message channels taken together.

A free terminal becomes a pre-calling terminal when, in response to aninstruction from the user peripheral associated therewith, it preparesor emits a calling signal including the address of a precalled terminalfor the purpose of then setting up a call during which said precallingterminal and said precalled terminal become a calling terminal and acalled terminal respectively.

There follows a description in general terms of the various dispositionswhich have been found to be advantageous, in particular in the contextof the present invention, for implementing networks of this kind. Thesedispositions are adopted in the two networks given by way of example andthey will be better understood during the subsequent description ofthese networks in more concrete terms.

The terminals of the networks are similar to one another. That is whyonly one of them is considered when describing their internal structure.In a disposition that is common to the first previously-mentioned knownnetwork and to the two networks given as examples of the presentinvention, the terminal under consideration includes the followingitems:

an emitter (1) controllable at least in frequency to emit said emissionwave and to apply information-carrying modulation to said wave;

emission positioning means for positioning the emission frequency ofsaid emitter in a spectrum range of the network, by forming a stack ofemission frequencies which are variable and which follow one anotherstarting from a fixed base frequency and going to the top of the stack,and being separated from one another by spectrum distances that are notless than a predetermined frequency increment;

reception means for demodulating some of the waves received by theterminal under consideration; and

a control circuit (40) for controlling the emitter, the emissionpositioning means, and the reception means.

The emission positioning means of the terminals are such that theiremission frequencies form a sequence of frequencies at intervals withbottom limits in which each next frequency is separated from a precedingfrequency by an inter-terminal distance that cannot drop significantlybelow a predetermined frequency increment (DF) that is greater than saidmessage spectrum width. That is why the positioning means of theterminal in question include emission support means themselves includingthe following items:

an emission positioning mixer (31) for mixing the waves received by theterminal in question with a local emission positioning wave having alocal emission positioning frequency (F(2P-1)A) in the optical range andwhich is applied to a local input (31A) of the mixer by the emitter (1)of the terminal to represent its emission frequency (F(2P-1));

an emission positioning detector (10) fed by said emission positioningmixer to form electrical beat signals each resulting from the mixing ofsaid local emission positioning wave with one of said received waveswhich corresponds to said signal, at least one of said signalsconstituting an emission positioning beat signal (F(2P-2)BE) if that oneof said received waves which corresponds thereto constitutes an externalemission positioning wave (F(2P-1)B) defining a support frequency(F(2P-1)) of the terminal under consideration, the frequency of saidsignal constituting an emission positioning beat frequency(F(2P-1)A-F(F(2P-2)B) which defines a support distance(F(2P-1)-F(F(2P-2)) between said emission frequency (F(2P-1)) and saidsupport frequency (F(2P-2)), one of said received waves constitutingsaid external positioning wave only if said support distance is situatedin a predetermined support interval including said frequency increment(DF);

an emission position discriminator (11) for receiving said emissionposition beat signal and for responding thereto by providing an emissionposition signal representative of an emission position difference equalto the difference between said emission positioning beat frequency(F(2P-a)A-F(F(2P-2)B) and a reference emission positioning frequency(DF-2FS) such that eliminating said difference means that the differencebetween said emission frequency (F(2P-1)) and said support frequency(F(2P-2)) of the terminal under consideration is equal to said frequencyincrement (DF); and

emission position control means (40) which control said emissionfrequency of said terminal in response to said emission positiondifference signal to make said emission positioning beat signal equal tosaid emission positioning reference frequency.

If the terminal under consideration is at spectrum distances more orless equal to the increment DF both as a preceding terminal and as afollowing terminal, then the detector 10 delivers two emissionpositioning beat signals.

That is why, in an advantageous disposition, the emission positiondiscriminator (11) includes an emission positioning filter for selectingand superposing two of said emission positioning beat signals, saidfilter being set to a frequency equal to said emission positioningreference frequency and having a narrow passband centered on saidfrequency, said emission position signal being representative of thepower of the output signal from said filter, and said emission positioncontrol means (40) controlling said emission frequency (F(2P-1)) of saidterminal to deliver said maximum amplitude.

The reception means may be of the heterodyne type and may include thefollowing items:

a local oscillator (2) for providing a local reception wave which isoptical and whose frequency is controllable, constituting a localreception frequency (L(2P-1)) of the terminal under consideration;

a heterodyning mixer (32) receiving firstly said waves received by theterminal under consideration, such a wave constituting one of saidreception waves and having one of said reception frequencies (F(2P)) forthe terminal under consideration when the message carried by said waveis to be transmitted to said terminal, said mixer receiving secondlysaid local reception wave and mixing said wave with said received waves;

a heterodyning detector (20) fed by said heterodyning mixer to provideelectrical beat signals, one of which is a reception beat signal(K(2P)H) having a reception beat frequency which is the differencebetween said reception frequency and said local reception frequency andwhich is close to a predetermined intermediate frequency (FI);

a demodulation filter (22) for receiving said reception beat signal, fordemodulating it, and for providing a demodulated signal representativeof said information to be transmitted to the terminal underconsideration and carried by said reception wave;

a frequency discriminator constituting a reception positiondiscriminator (21) for receiving said reception beat signal and forresponding thereto by providing a reception position error signalrepresentative of the difference between said reception beat frequencyand said intermediate frequency (FI); and

reception position control means (40) which controls said localreception frequency in response to said reception position error signalto make said reception beat frequency equal to said intermediatefrequency.

The control circuit of the terminal under consideration is connected tothe user peripheral associated with said terminal to receive therefrom amessage to be transmitted. It includes modulation means which controlthe emitter of said terminal accordingly to provide saidinformation-conveying modulation of the emission wave when the terminalunder consideration is emitting. It includes said emission positioncontrol means. It receives said demodulated signal and includesaddress-recognition means for recognizing the address of the terminalunder consideration. It switches the terminal to receive mode when saidaddress has been recognized and it subsequently transmits the receivedmessage to the associated user peripheral. It receives said receptionbeat signal and it includes said means for controlling receptionposition.

In addition, the terminal under consideration is suitable for emittingand receiving signalling in the form of modulation which affects opticalcarrier waves that are guided by said emission and reception fibers ofsaid terminal and which are transmitted via said star coupler. Suchsignalling includes a calling signal which is emitted by the terminalwhen it is precalling and which includes the address of the precalledterminal.

In an advantageous "stack compression" disposition, the control circuitof the terminal under consideration is permanently ready, i.e. at allinstants throughout the duration of a call, to vary the emissionfrequency of the terminal.

Such variation is performed if a support frequency lower than theterminal (as defined below) moves towards the base frequency. Under suchcircumstances, the emission frequency of the terminal underconsideration accompanies the displacement of said support frequency.Such a variation is also performed if such a lower support frequencythat was previously in use disappears. Under such circumstances, theemission frequency moves progressively towards the base frequency untila new lower support frequency can be defined. The new lower supportfrequency may be the emission frequency of a preceding terminal if sucha terminal exists, or it may be the base frequency itself.

The emission positioning means of the terminal under considerationenable it to perform these functions by means of the above-mentioneditems except that the emission position control means include stackcompression means for this purpose which may be incorporated in thecontrol circuit of the terminal and which are as follows:

initial emission positioning means which are put into action when theterminal under consideration becomes free or at least before thebeginning of a new call, these means bring the current emissionfrequency of the terminal under consideration to the top of the stack ofthe emission frequencies of the various terminals; for example they maymove said frequency progressively away from the previously used emissionfrequency until they detect that said emission frequency has indeedreached the top of the stack;

lower support detection means for detecting the possible presenceamongst the waves received by the terminal under consideration of anexternal lower emission positioning wave constituted by one of saidexternal emission positioning waves defining a lower support frequencyfor the terminal; such a frequency is called a "support" frequency forthe terminal providing said support frequency is closer to the basefrequency than is the emission frequency of the terminal; said meansprovide a loss-of-lower-support signal when such an external loweremission positioning wave no longer exists;

means for servo-controlling the emission position during a call whichare kept in action during the call so long as there exists one suchlower support frequency for the terminal under consideration; said meansare called "emission support means" and they control the emissionfrequency of said terminal in the manner indicated above to eliminatesaid difference in emission position; and

emission shift means which are put into action during a call when one ofsaid loss-of-lower-support signals is provided for the terminal underconsideration; these means move the emission frequency of the terminalprogressively towards the base frequency until saidloss-of-lower-support signal is no longer provided.

This moving together of frequencies is performed at a shift speed whichis preferably chosen to be large. However this speed must be smallenough to enable the emission position servo-control means of afollowing terminal to follow the variation in said frequency. This isnecessary because said frequency constitutes the lower support frequencyfor the following terminal.

A stack of emission frequencies is thus achieved for the variousterminals building up from the base frequency. These frequencies followone another at intervals, many of which are normal intervals equal tosaid frequency increment. A larger frequency interval appears when aterminal stops transmitting a message. Under such circumstances, thefollowing terminal can no longer define a lower support frequency. Ittherefore progressively and regularly shifts its emission frequencytowards the base frequency during a shift period which constitutes afraction of said call period and which comes to an end either when a newlower support frequency can be defined or else when the call is over. Ifthe frequency shift speed is chosen to be large enough, the shift periodis small enough for large frequency gaps increasing the total width ofthe spectrum range required by the network to be kept to a small amountonly.

In another advantageous disposition, said lower support detection meansfor the terminal under consideration comprise the following items:

non-selective support detection means for supplying a loss-of-supportsignal when one of said external emission positioning waves for theterminal no longer exists; for this purpose, these means may monitorsaid emission position signal which is representative of the power ofthe emission positioning beat signal; a predetermined drop such as arapid drop of about 50% in said power indicates either that an externallower emission positioning wave has disappeared or else that an externalupper emission positioning wave has disappeared; under suchcircumstances, said means trigger the appearance of the loss of supportsignal; and

support discrimination means which are put into operation at least whensaid loss-of-support signal is provided and which then provide saidloss-of-lower-support signal if the external emission positioning wavethat has disappeared is a lower one of said external emissionpositioning waves.

The two networks given by way of example differ from each other as tothe structure of said support discrimination means. The first networkoperates in duplex mode. As a result, while the terminal underconsideration is engaged in a call, it uses two adjacent emissionfrequencies and the terminal in question is emitting either on the lowerfrequency or on the upper frequency of the call. It thereforeconstitutes either the lower terminal or the upper terminal of the pairof terminals that are communicating with each other, it being understoodthat the lower terminal is the terminal that comes before the otherterminal in said pair in the sequence of the terminals that correspondsto the stack of emission frequencies. In the first network given by wayof example, the lower terminal is the calling terminal, but it will beunderstood that it could equally well be the called terminal.

In an advantageous disposition, the support discrimination means of theterminal under consideration in a network operating in duplex moderecords a relative position bit at the beginning of each call andretains it until the end of the call, said bit representing the factthat the terminal under consideration is calling or called and isconsequently the lower terminal or the upper terminal in the pair ofterminals concerned by a given call. These means provide a lowindication if the terminal is the lower terminal of the pair. The saidloss-of-lower-support signal is provided when said loss-of-supportsignal and said low indication are provided simultaneously. As a result,the pair of terminals under consideration, i.e. the pair including theterminal under consideration during a call under consideration, willoffset its two emission frequencies towards the base frequency if thepreceding call disappears, while, in contrast, the two emissionfrequencies of the pair under consideration will remain unchanged if itis the following call that disappears.

It should be understood that in practice the various means included inthe control circuit are advantageously implemented in the form ofprogram elements incorporated in said circuit.

In another advantageous disposition, the network includes a markinggenerator for prior marking at a marking instant one or more spectrumpositions which are marked for use during the next call. These markedpositions are spectrum positions which are not occupied by any of theterminals in the network at the marking instant and which are to beseized by one or more of the terminals to implement the next call, i.e.the first call to begin on the network after said instant.

Such prior marking has the advantage of reducing the access time of thenetwork.

The structure of the marking generator may be analogous to that of theterminal under consideration except that no information-carryingmodulation is required on the optical waves that it emits since the onlyfunction of these waves is to define frequencies.

It includes the following items:

an emission fiber (B51) and a reception fiber (B52) for connecting it tosaid star coupler (CE) like the terminal under consideration;

at least one marking emitter (B2) that is controllable in frequency toemit an optical marking wave (FM1) on said emission fiber; and

marking positioning means (B22, B32, B70) controlling the markingemitter to give the marking wave a marking frequency (FM1) having apredetermined relationship with a spectrum position that is to be seizedto establish the next call by means of a terminal participating in saidcall, which position is thus marked by said marking wave.

The predetermined relationship is such that the marking wave facilitatesand/or accelerates displacement of the emission frequency which theterminal participating in the call must perform to reach the markedspectrum position.

By way of examples of such predetermined relationships, the markingfrequency may coincide with the marked position or it may be situated ata predetermined spectrum distance from said marked position, whichdistance is related to said frequency increment.

Naturally, when the network includes the above-mentioned stackcompression means, the position(s) marked by the marking generator(s)is/are situated at the top of the stack, as described below.

In another advantageous disposition, the network includes a signallingreference generator which itself includes:

a signalling emitter (B4) which is controllable in frequency to emit anoptical wave that constitutes a signalling reference wave at asignalling frequency (FZ);

signalling positioning means including signalling position servo-controlmeans and signalling position shift means for positioning saidsignalling frequency in the spectrum range of the network;

an emission fiber (B51) for guiding said signalling reference wave tosaid star coupler (CE); and

a reception fiber (B52) for guiding optical waves that have beentransmitted to the coupler and which are received by said generator.

The signalling position servo-control means are analogous to the controlmeans of the terminal under consideration and include the followingitems:

a signalling positioning mixer (B24) for receiving firstly said wavesreceived by the signalling reference generator and secondly a localsignalling positioning wave that is optical and that defines saidsignalling frequency;

a signalling positioning receiver (B34) fed by said signallingpositioning mixer to form electrical beat signals one of which is asignalling positioning beat signal that results from mixing said localsignalling positioning wave with an external signalling positioning waveconstituted by one of said received waves defining a signalling supportfrequency which is the most distant frequency in the stack of saidemission frequencies, the frequency of said signal constituting asignalling positioning beat frequency lying in a predetermined range andrepresentative of a spectrum distance between said signalling frequencyand said signalling support frequency;

a signalling position discriminator (not shown) for receiving saidsignalling positioning beat signal and for responding thereto byproviding a signalling position signal representative of the differencebetween said signalling positioning beat frequency and a predeterminedfrequency; and

signalling frequency control means (B70) which control said signallingfrequency in response to said signalling position signal toservo-control the spectrum distance between said signalling frequencyand said signalling support frequency to a predetermined spectrumdistance supplement which constitutes a signalling supplement.

The signalling position shift means move the signalling frequency awaywhen a new call is established, with this being done over a shiftdistance that is equal to a number of frequency increments equal to thenumber of emission frequencies that are used by the new call.

The emission positioning means of the terminal under consideration theninclude, in association with its said control circuit:

signalling search means for moving its emission frequency progressivelyaway from the base frequency when the terminal becomes free, and forrecognizing when said emission frequency coincides with said signallingfrequency; and

standby servo-control means controlled by said signalling search meansto servo-control said emission frequency to said signalling frequencyfrom the moment when coincidence between said two frequencies isrecognized.

This disposition has the advantage that the access time to the networkis reduced by the fact that when the terminal under consideration isfree, its spectrum position is at least close to the position that itwill occupy during its next call.

The control circuit (40) of the terminal under consideration furtherincludes call signalling means for modulating said emission wave havingsaid signalling frequency with call signalling and for emitting saidsignalling when a message is to be transmitted by said terminal, thefrequencies of the emission wave as modulated in this way occupying asignalling channel (KZ) having a signalling spectrum bandwidth.

The terminal under consideration under includes reception standbyinitial positioning means (B70) for bringing said local frequency ofsaid terminal to a spectrum position whose distance from said signallingfrequency is equal to said intermediate frequency when said terminal isfree so as to enable said reception means subsequently to receivemessages in the signalling channel (KZ).

The said emission positioning means of the terminal under considerationinclude emission search means which are put into operation when theterminal is to switch to emission and which move its emission frequencytowards said base frequency from said signalling frequency down to apre-emission position in which one of said lower support frequenciesappears for said terminal, and reception search means which are put intooperation by said control circuit when said terminal is to switch toreception and which bring said local reception frequency of saidterminal to a pre-reception position whose distance from thepre-emission position of one of said terminals is equal to saidintermediate frequency.

The pre-emission position used for establishing said pre-receptionposition of the terminal under consideration is the pre-emissionposition of the same terminal if the call is being established inalternating mode. If the call is being established in duplex mode, thepre-emission position used is the pre-emission position of the terminalwhich is paired with the terminal under consideration.

In another advantageous disposition, the signalling reference generatoris included in the control unit BG. The control unit also includesreception means similar to said reception means in the terminal underconsideration and a control circuit which is analogous to said controlcircuit of the terminal under consideration except that firstly it doesnot provide communication with an associated user peripheral andsecondly it is required to receive, to process, and to emit signallingover the signalling channel for the purpose of controlling the network.

In another advantageous disposition, the signalling reference generatorconstitutes one of said marking generators and for this purpose itfurther includes:

at least one first marking emitter (B2) of controllable frequency foremitting a first marking wave on said emission fiber of said generator;and

first marking positioning means for giving said wave a first markingfrequency (FM1).

Said first marking positioning means are analogous to the emissionpositioning means and include the following items:

a first marking positioning mixer (B32) for receiving firstly said wavesreceived by the signalling reference generator and secondly a localfirst marking positioning wave that is optical and that defines thefirst marking frequency;

a first marking positioning detector (B22) fed by the first markingpositioning mixer to form beat signals that are electrical, one of whichis a first marking positioning beat signal resulting from mixing saidlocal first marking positioning wave with one of said received wavesthat defines said signalling support frequency (F2P), the frequency ofsaid signal constituting a first marking positioning beat frequencylying in a predetermined interval and representative of an intervalbetween said first marking frequency and said signalling supportfrequency; and

a first marking positioning discriminator (not shown) for receiving saidfirst marking positioning beat signal and for responding thereto byproviding a first marking position signal representative of thedifference between said first marking position beat frequency and apredetermined frequency.

The control circuit (B70) of the signalling reference generatorgenerates the first marking frequency in response to the first markingposition error signal to servo-control the spectrum distance betweensaid first marking frequency and the signalling support frequency (F2P)to said frequency increment (DF).

When the terminal under consideration is to switch to reception while incommunication with another terminal which is positioned on said firstmarking frequency, the first marking wave enables the terminal underconsideration to servo-control its local reception frequency relative tosaid first marking frequency without needing to wait for an emissionfrom said other terminal for this purpose. If the marking frequency isunique, said signalling supplement is advantageously equal to twice saidfrequency increment.

When operating in duplex mode where two adjacent message channels areused for each call and where the terminal under consideration is theupper terminal, i.e. is to occupy the upper channel, i.e. the channelwhose carrier frequency is further from the base frequency, said firstmarking frequency also constitutes said lower support frequency for saidterminal.

Under such circumstances, a second marking frequency (FM2) follows thefirst at a distance equal to said frequency increment. A correspondingsecond marking wave is emitted by a second marking emitter (B3) providedwith spectrum positioning means analogous to the above means. It enablesthe signalling frequency and the local reception frequency of the upperterminal to be positioned.

The signalling supplement is then advantageously equal to three timessaid frequency increment.

In another advantageous disposition the means for servo-controlling theemission position of the terminal under consideration further includespectrum positioning assistance means themselves including:

a positioning assistance generator (53) for providing a positioningassistance signal at a predetermined positioning assistance frequency(FS); and

positioning assistance modulation means (1) for modulating at least afraction of the emission wave from the terminal under consideration bysaid positioning assistance signal so as to generate two positioningassistance side waves at two positioning assistance side frequencieswhose spectrum distances from the emission frequency (F(2P-1)) of saidterminal are equal to said positioning assistance frequency (FS). Afirst side wave has a first positioning assistance side frequency(F(2P-1)A) closer to said lower support frequency (F(2P-2)) of saidterminal than its emission frequency. It consitutes said local emissionpositioning wave transmitted to said emission positioning mixer of theterminal under consideration.

The second positioning assistance side wave formed in this wayconstitutes said external emission positioning wave for the terminalfollowing the terminal under consideration.

The positioning assistance frequency (FS) preferably lies betweenone-fourth and one-half of said frequency increment (DF).

These positioning assistance means present the advantage that theemission positioning beat frequency may be much less than the supportdistance. It may be situated, for example, in the microwave rangewhereas the support distance is substantially equal to the frequencyincrement which is situated in the optical frequency range. The emissionpositioning beat signal can thus be processed by means of well-knownelectronic components of acceptable price. Given the presence of thesepositioning assistance means, it will be understood that when it isspecified above that a wave, a signal, or a first frequency defines asecond frequency or a spectrum distance such as a support distance, thatmeans that the second frequency or distance defined in that way is equalto the defining frequency plus or minus a predetermined value, saiddefining frequency being the frequency of said wave or said signal orbeing said first frequency, said predetermined value being equal to oneor two times the positioning assistance frequency.

In the two networks given by way of example, the frequency(F(2P-1)A-(F(2P-2)B) which defines the support distance(F(EP-A)-F(EP-2)) is equal to said distance minus twice the positioningassistance frequency (FS). Given the fact that said support distance issubstantially equal to the frequency increment (DF), the frequency whichdefines the support distance is substantially equal to DF-2FS.

Another advantageous disposition concerns the use that may be made of asignalling channel that includes the signalling frequency and all of thefrequencies occupied by the various signalling transmitted over thenetwork to set up and control calls, and more generally to control thenetwork.

Such signalling includes the following in particular:

calling signalling emitted by a precalling terminal and containing theaddress of the precalled terminal;

call acknowledge signalling emitted by the precalled terminal inresponse to the calling signalling to inform the precalling terminalthat the precalled terminal is ready to set up a call; and

signalling to allocate a call entitlement, which signalling is used whentwo terminals seek to establish two different calls simultaneously onthe same channel, with the signalling then defining which call haspriority.

The advantage of this disposition appears when many calls but not allcalls need to transmit messages in limited time, with the messages eachincluding a large quantity of data, which messages may be called"heavyweight". During "heavyweight" calls, the high data rate requiresmessage channels of large bandwidth, whereas during other more"lightweight" calls require less bandwidth. In this disposition, thesignalling channel is shared between the various network terminals notonly for transmitting said signalling but also for transmitting"lightweight" messages, i.e. messages each including a relatively smallamount of data.

More precisely, the terminal under consideration includes:

means for classifying messages to indicate whether a message to betransmitted is a heavyweight message that needs to be transmitted at arelatively high data rate over a relatively long call duration orwhether the message is a lightweight message that can be transmitted ata relatively low data rate and/or during a relatively short callduration;

heavyweight message transmission means for transmitting said heavyweightmessages in said message channels; and

lightweight message transmission means for transmitting said lightweightmessages in said signalling channel which then constitutes a channelthat is shared between said terminals.

The message classification means are incorporated in the control circuit40.

The heavyweight message transmission means of the terminal underconsideration are the means as described above and they come into actiononly when the terminal is busy. Its lightweight message transmissionmeans are the same means except that they are put into action by itscontrol circuit while the terminal is in a situation described above asbeing that of a free terminal. They are put into action when the userperipheral associated with the terminal seeks to emit a message and themessage is classified as being lightweight.

This disposition makes it possible, without excessively increasing thebandwidth of the signalling channel, to considerably reduce the numberof message channels when the number of heavyweight messages to betransmitted per unit time is significantly smaller than the total numberof messages. The spectrum available to the network is thus used moreefficiently.

In another advantageous disposition, the bandwidth of the shared channelis substantially equal to said message bandwidth.

The electronic components used by the shared channel can then besubstantially the same as those used by the message channels.

In another advantageous disposition, the shared channel is timemultiplexed between said terminals or at least between those of saidterminals that are not busy transmitting a heavyweight message, witheach of the terminals transmitting lightweight messages and signallingthat it needs to transmit over the time-shared channel in the time slotsallocated thereto during each multiplexing cycle.

A protocol for gaining access to the shared channel is provided. It maybe similar to the protocols that are already known for gaining access toa signalling channel. Call entitlements, including entitlements toestablish heavyweight calls are allocated in centralized manner in thecontrol unit BG which dialogs for this purpose with the terminals overthe signalling channel. It will nevertheless be understood that suchallocation could also be achieved in decentralized manner in whichpriority calls are defined by dialog between the terminals over thesignalling channel.

There follows a more concrete description of the terminal underconsideration belonging to the first network given by way of example. Itmay be the terminal T1 as shown in FIG. 1, for example. A star couplerof the network is designated by reference CE.

An emitter 1 and a local oscillator 2 of the terminal underconsideration are constituted by means of at least two semiconductorlasers whose frequencies are controlled electrically. An emissionfrequency control current is provided by said emission positioning meansvia a regulator circuit 41.

A current controlling the emitted optical intensity includes threecomponents:

a DC bias component delivered by the regulator circuit 41;

an information-carrying modulation component including relatively lowfrequencies and constituted by a binary digital signal representative ofthe data that the user peripheral 50 associated with the terminaldesires to transmit; this signal is formed in a circuit 52 connected tothe user peripheral; and

a positioning assistance modulation component which is sinusoidal and ata frequency higher than the frequencies of the information-carryingmodulation component, which component constitutes a positioningassistance frequency FS. This third component is applied to the emitter1 by a positioning assistance generator 53.

An optical switch 34 serves to isolate the emitter from an emissionfiber 61 while the terminal is being positioned. An optical coupler 33takes a portion of the wave delivered by the emitter to mix it inanother optical coupler 31 with the waves received over a receptionfiber 62. The coupler 31 constitutes said emission positioning mixer.The resulting mixed waves are detected by an emission positioningdetector 10 of the quadrature type. The resulting electrical signalconstitutes said emission positioning beat signal. Its frequency ismeasured by an emission position discriminator 11 which is centered onthe frequency DF-2FS. The discriminator delivers an emission positionerror signal for use in servo-controlling the emitter.

A heterodyning detector 20 detects the waves leaving a heterodyningmixer which is constituted by an optical coupler 32 and which receivesthe waves received over the reception fiber 62 and the wave emitted bythe local oscillator 2. A reception position and frequency discriminator21 centered on an intermediate frequency FI generates a receptionposition error signal enabling the local oscillator 2 to beservo-controlled. Information-extracting demodulation is performed onthe signal detected at 20. It is performed by an amplifier circuitconstituting a demodulation filter 22 centered on the intermediatefrequency FI and by a regenerator 51 which reconstitutes the digitaldata in baseband for the user peripheral 50.

Overall control of the terminal is provided by a control circuit 40which is constituted by a microcircuit and which is connected to theuser peripheral 50 associated with the terminal.

The control circuit 40 has inputs which receive the error signals asdigitized by an analog-to-digital converter 43. A memory 42 contains, inparticular, the operating ranges of the lasers in the emitter 1 and theoscillator 2, i.e. a set of current values describing the tuning rangeof each of these lasers in steps. The circuit 40 generates referencesignals and transmits them to a regulator circuit 41 which regulates thecurrents and the temperatures of the lasers. It also controls the stateof the switch 34 connecting the emitter to the star coupler CE.

For this purpose, it is provided with software constituting theabove-mentioned means and providing the following functions inparticular:

calculating the tuning currents for the emitter and the local oscillatoras a function of the position error signals in such a manner as tomaximize the signal delivered by the discriminator 11 and to cancel theerror measured by the discriminator 21, with the position of theterminal in the stack or above the stack having no effect on thisservo-control;

implementing stack compression, i.e. detecting the disappearance of thecall beneath by observing the variations in the power of the emissionpositioning beat signal and the 2xDF shift instruction which must resulttherefrom;

taking account of control information from the user peripheral: callrequests including the address of the called terminal, alarm signals,and security signals specifying an incorrect transmission that should beinterrupted and subsequently restarted;

taking account of signalling coming from the control unit when theterminal is on the shared channel, and in particular authorizations toestablish wideband calls, i.e. so-called "heavyweight" calls that needto be established over a message channel; and

setting up a call, and in particular if the call is a heavyweight calllowering the frequencies of the lasers from the signalling frequencydown to the positions they are to occupy during the call in compliancewith the process described above.

The network includes a control unit BG which includes, in particular,the above-described base generator, marking generators, and signallingreference generator. As shown in FIG. 3, the control unit contains fivelasers as follows:

a base generator B1 which is stabilized absolutely and which emits abase wave at a base frequency F0 to constitute a reference at the bottomof the spectrum range of the network;

two emitters B2 and B3 which are stabilized in relative manner and whichemit the first and second marking waves to constitute two references atthe top of the stack; and

a signalling emitter B4 and a local oscillator B5 for use in dialoguingwith free terminals over the shared channel.

A coupler B50 serves to inject the waves emitted by the sources B1, B2,B3, and B4 into an emission fiber B51 which is connected to the starcoupler CE.

The frequency FM1 of the first marking emitter B2 is servo-controlled(see FIG. 4) to a distance DF above the last message channel K(2P). Thefrequency FM2 of the second marking emitter B3 is servo-controlled at adistance DF above the first marking frequency. The frequency of thesignalling emitter B4 is servo-controlled to a distance DF above thesecond marking frequency. The emitters B2, B3, and B4 are subjected topositioning assistance modulation at the frequency FS so that theservo-control used can be identical to that used for the emitter of theterminal under consideration. For this purpose, a sinewave signal isdelivered at the frequency FS by a positioning assistance generator B60.

Optical couplers B12, B13, and B14 are used to take fractions of thewaves emitted by the emitters B2, B3, and B4 for the purpose ofproviding the beats required for servo-control purposes. Couplers B22,B23, B24, and B25 perform the following mixing operations respectively:

mixing the first marking wave from the emitter B2 with the signal from areception fiber B52 for servo-controlling said emitter;

mixing the second marking wave from the emitter B3 with the firstmarking wave for servo-controlling said emitter;

mixing the signalling reference wave from the emitter B4 with the secondmarking wave to servo-control said emitter; and

mixing the wave from the local oscillator B5 with the signal from thecoupler B22 to servo-control said local oscillator.

Positioning receivers B32, B33, B34, and B35 provide position errorsignals E2, E3, E4, and E5 for these four servo-control operations. Eachof the receivers B32, B33, and B34 is constituted by a quadraticdetector and a filter centered on the frequency DF-2FS. The receiver B35is constituted by a frequency discriminator centered on the intermediatefrequency FI.

A control circuit B70 has the following functions:

it performs the servo-control operations, for which purpose it receivesthe position signals E2, E3, E4, and E5 for the lasers B2, B3, B4, andB5 respectively;

it performs the protocol for gaining access to broadband communicationsby dialoguing with free terminals over the signalling channel, receivingmessages D5 from these terminals via a demodulator B61, and transmittingmessages D4 to these terminals by frequency modulating the signallingwave emitted by the emitter B4, thereby taking account of the variousrequests for broadband access and transmitting authorizations to set upcalls; and

it increases the frequencies of the lasers B2, B3, B4, and B5 by 2DFafter a broadband call has been established so as to mark the new top ofthe stack taking support from the last emission frequency.

There follows a description of the setting up of a call and then theclearing down of a call.

Two free terminals are put into communication over the signallingchannel K7. The control unit is also present on this channel. It isaware of current call requests, of such priorities as may exist, and ofhow the spectrum range of the network is occupied. As a function of thisinformation it allocates call entitlements over the message channelsmarked at the top of the stack and over the shared channel.

During the time taken by the terminals concerned to position themselveson these channels, the waves emitted by these terminals must not beinjected into their emission fibers in order to avoid disturbing othercalls passing through the star coupler CE. For this purpose, theemitters are temporarily disconnected from the coupler by the switchessuch as 34.

The calling terminal:

reduces the frequency of its emitter by two 2×DF and it servo-controlsit to DF above the highest emission frequency F(2P); and

reduces the frequency of its local oscillator by DF and servo-controlsit to F1 below the second marking frequency LM2.

The called terminal:

reduces the frequency of its emitter by DF and servo-controls it to DFabove the first marking frequency LM1; and

reduces the frequency of its local oscillator by 2×DF to servo-controlsit to FI below the first marking frequency.

The emitters are reconnected to the star coupler and the control unitincreases the marking frequencies by 2×DF to servo-control them on thenew top of the stack. All of the free terminals are servo-controlled tothese frequencies so they follow this movement.

When a call is cleared down, the two terminals engaged in the calldisconnect their emitters from the star coupler. In each of theseterminals, the frequency of the local oscillator is then temporarilyservo-controlled on the emitter. The frequency of the emitter isincreased to the top of the stack which it recognizes by detecting thetwo marking waves. The local oscillators and emitters can then beservo-controlled again on the signalling frequency. The calling terminalof the next call up detects that emission from the terminal beneath ithas disappeared and shifts its emission frequency progressively through2×DF to fill up the gap released by the call that has finished. All ofthe higher channels in the stack including the signalling channel followthis shift.

FIGS. 4 to 9 show the frequency spectrum of the network when a number Pof calls are in progress, with the number 2P being less than the numberN of the terminals. These figures show, in particular, a top pair ofchannels constituted by message channels K(2P-1) and K(2P) used by thetop pair of terminals, i.e. the pair of terminals whose emissionfrequencies F(2P-2) and F(2P) are the furthest from the base frequencyF(O).

These two channels are shown with two different types of shading, with agiven type of shading being used for all of the waves emitted by a giventerminal and for all of the electrical signals that result therefrom.The top pair is the pair conveying the call that has most recently beenset up on the network. That is why it is at the top of the stack. Moreprecisely, the calling channel K(2P-1), i.e. the channel used by thecalling terminal, constitutes the lower channel of said top pair, withthis terminal thus constituting the lower terminal of this pair ofterminals. In similar manner, the channel called K(2P) constitutes theupper channel of the same pair.

The emission frequencies of these two terminals are written respectivelyF(2P-1) and F(2P). These frequencies are the mid-frequencies of thecorresponding channels.

The local reception waves L(2P-1) and L(2P) which are generated in thesetwo terminals respectively are represented beneath the frequency axis tofacilitate understanding the drawing. The message channels occupied byother terminals are shown without shading, and in particular the upperchannel K(2P-2) occupied by the upper terminal of the top-but-one pairof terminals.

The emission frequency F(2P-2) which is situated in the middle of thechannel K(2P-2) constitutes the lower support frequency for the terminaloccupying the channel K(2P-2). The mid-frequency F(2P-1) of this channelconstitutes the lower support frequency for the terminal occupying thechannel K(2P).

In general, the frequency differences established over the network arerepresented by heavy horizontal arrows, with the arrow heads designatingfrequencies that are controlled or predetermined to ensure saiddifferences.

Emission frequency servo-control is generally bilateral servo-control.That is why the servo-control arrows such as the arrow betweenfrequencies F(2P) and F(2P-1) are generally two-headed arrows.

Emission frequency F(1) which is situated at the bottom of the stack isservo-controlled unilaterally relative to the base frequency F(0). Thedistances between terminals servo-controlled in this way are equal tothe frequency increment DF, with the distance between the frequenciesF(1) and F(0) being the positioning assistance frequency.

The signalling channel is shown at K2. The frequencies of the signallingreference wave and of the local reception wave of the signallinggenerator BG are shown at FZ and LZ, and the first and second markingfrequencies are shown at FM1 and FM2, respectively.

The servo-control of the frequencies F(2P), FM1, and FM2 is bilateral.The servo-control of the frequency F2 is unilateral.

FIG. 5 shows the message channels K(2P-2), K(2P-1), and K(2P), togetherwith their mid-frequencies F(2P-2), F(2P-1), and F(2P). This figure alsoshows said first and second positioning assistance side frequenciesF(2P-1)A and F(2P-1)B of the terminal under consideration which occupiesthe channel K(2P-1). These frequencies are respectively equal toF(2P-1)-FS and F(2P-1)+FS.

For the channel K(2P-2) the corresponding waves are shown at (F(2P-2)A)and (F(2P-2)B), respectively.

The lower support distance F(2P-1)-F(2P-2) of the terminal occupying thechannel K(2P-1) is maintained equal to the frequency increment DFbecause the difference between the frequencies F(2P-1)A and (F(2P-2)B)is maintained equal to said emission positioning reference frequencywhich is equal to DF-2FS.

FIG. 6 shows the spectrum of the same optical waves at the inlet to thedetector 10 of the terminal under consideration. It differs from FIG. 5by the waves from the emitter 1 being of increased relative intensity,i.e. the waves occupying the channel K(2P-1) and the side frequencies(F(2P-1)A) and (F(2P-1)B).

FIG. 7 is a diagram showing a theoretical spectrum of the electricalsignals that result from detecting said waves in the detector 10. Thepassband of the discriminator 11 is centered on the positioningreference frequency DF-2FS and is shown at A11. The signals resultingfrom detecting some of the waves are designated by the references forsaid waves plus the letter E. Their frequencies are equal to thedifference between the frequencies of said waves and the firstpositioning assistance side frequency (F(2P-1)A) of said terminal whichis one of the higher power frequencies. It will be understood that somesignals have been omitted such as those that result from beats ofvarious waves with the channel K(2P-1) or with the frequency (F(2P-1)B)which also correspond to high powers. It will also be understood thatsome signals such as K(2P)E are shown to facilitate understanding butthat they are at frequencies which are too high to appear in fact inelectrical form at the outlet from the detector.

FIG. 8 represents the optical spectrum at the inlet of the terminal 20in the terminal under consideration. In this spectrum the highest powerwave is the local reception wave coming from the local oscillator 2 andits frequency is designated by reference L(2P-1) and is equal toF(2P)-FI because of the servo-control performed by means of the detector20.

FIG. 9 shows the electrical signals resulting from some of these wavesbeing detected by the detector 20, with the electrical signals beingdesignated by the same references as said waves plus the letter H. Theirfrequencies are equal to the difference between the frequencies of saidwaves and the high power frequency L(2P-1). The same remarks apply asapplied to FIG. 7.

The passbands of the discriminator 21 and of the amplifier and filtercircuit 22 are shown as Q21 and Q22 respectively.

The second network constituting an embodiment of the present inventionis now described. This network is shown in FIG. 10. It is generallysimilar to the above-described first network except that it operates inalternating mode and compared with the first network it has dispositionsthat differ at least in part and that are adapted to this mode ofoperation, with some of them now being described.

Firstly, and in general, it is advantageous for said supportdiscrimination means of the terminal under consideration in a networkoperating in alternating mode to include means for forming the supportdiscrimination wave that are put into action by said loss-of-supportsignal to provide a support discrimination wave to said local input(Y31A) of said emission positioning mixer (Y31) of the terminal, whichsupport discrimination wave has a support discrimination frequency at asupport discrimination spectrum difference from a value of said localemission positioning frequency immediately prior to said loss-of-supportsignal being formed that is suitable to cause a positioning differenceto appear affecting the frequency of said emission positioning beatsignal, said positioning difference being capable of having one or otherof two algebraic signs, positive and negative; saidloss-of-lower-support signals being provided when said positioningdifference has a predetermined algebraic sign.

In another advantageous disposition, the support discrimination waveforming means are means for giving rise to an emission difference thattemporarily affects the emission frequency of the terminal underconsideration. This emission difference may be positive, for examplei.e. the emission frequency may be moving temporarily a little furtherfrom the base frequency. When the positioning assistance frequency FS isless than half the frequency increment DF, the loss-of-lower-supportsignal will be provided and the emission frequency will be shiftedprogressively towards the base frequency if the emission positioningbeat signal is subjected to a negative frequency difference. This may bedetected, for example, by a control circuit Y40 at the output of asupport discriminator Y12 constituted by a frequency discriminatorprovided for this purpose and fed from the output of the emissionpositioning detector Y10.

As shown in FIG. 10, the terminal under consideration of the networkoperating in alternating mode includes items analogous to those of theterminals shown in FIG. 2 except that the support discriminator Y12 isadded and the control circuit Y40 is partially modified. When two itemsin these two terminals are analogous, the reference symbols designatingthese two items are the same except that the letter "Y" is added to thebeginning of the reference symbols used for items in the terminal shownin FIG. 10.

We claim:
 1. A method for maintaining the frequency of an emissionsignal at a given spectrum distance from a support frequency of a givenexternal signal, wherein said emission signal is amplitude modulated ata predetermined frequency to generate a side signal having apredetermined offset in frequency relative to said emission signal, saidside signal and said external signal are mixed to form a beat signal,and the frequency of said emission signal is controlled in response tovariations in the frequency of said beat signal.
 2. The method of claim1, wherein said support frequency is an optical frequency.
 3. Apparatusfor maintaining an emission frequency at a given spectrum distance froma given support frequency, said apparatus comprising:an emitter foremitting an emission wave at said emission frequency; modulation meansto apply an amplitude modulation at a predetermined positioningassistance frequency to at least a fraction of said emission wave,thereby providing at least one first positioning assistance side wavewhose frequency is at a predetermined spectrum distance from saidemission frequency equal to said positioning assistance frequency; apositioning mixer mixing said first positioning assistance side wavewith an external emission positioning wave representative of saidsupport frequency to form a positioning beat signal; and means forcontrolling said emission frequency in response to variations in thefrequency of said positioning beat signal so as to position saidemission frequency at said given spectrum distance from said supportfrequency.
 4. Apparatus according to claim 3, wherein said positioningassistance frequency is less than a frequency increment whichconstitutes said defined spectrum distance.
 5. Apparatus according toclaim 3, wherein said modulation means are first modulation means, saidapparatus further including second modulation means for applyingmodulation at a second positioning assistance frequency to at least afraction of a support wave having said support frequency, therebyforming two side waves one of which constitutes said external emissionpositioning wave and has a frequency which is closer to said emissionfrequency than said support frequency and which is situated at aspectrum distance from said support frequency equal to said secondpositioning assistance frequency.
 6. Apparatus according to claim 5,wherein said second positioning assistance frequency is equal to saidpositioning frequency, said frequencies being less than half saidfrequency increment.
 7. Apparatus according to claim 6, wherein saidpositioning assistance frequencies are greater than one-fourth of saidfrequency increment.
 8. Apparatus according to claim 3, wherein saidmodulation by a positioning assistance frequency means forms twopositioning assistance side waves, disposed in the spectrum on eitherside of said emission wave, said first positioning assistance side wavebeing that one of these two waves which is closer to said supportfrequency.